Thanks for the reply Aragorn.
I have just changed the 'Guest' settings on PCB and can now connect to those shares from PCA but I am almost sure I had the Guest account disabled on both computers before this problem arose. That account is enabled on both computers now, but that may be because I have removed and then re-enabled simple file sharing in an effort to solve the problem.
If anyone has changed the Guest password then it is me because I am the only one using the computers
If I wish to disable the Guest account do I need to move away from simple file sharing?
TIA